# Speed of light

is it possible to catch light particals and have them push you along in some kind of space ship, and if you could would it be the speed of light

No, but apparently swans make good propulsion for early space craft. Light particles or "Photons" are packets of energy that you can't catch. If you were to try and "grab" one with your hand and catch a ride you may be disappointed. Traveling at light speed is impossible for such a large mass as yourself, but seriously swans all the way...

russ_watters
Mentor
Actually, every time a photon hits you and you absorb it, you have "caught" it. And when you do, it transfers its momentum to you. So yes, you could propel a spaceship that way (actually, reflecting them is better...), but you'd need an enormous solar sail to catch enough photons to be useful. And no, you could never reach the speed of light. You could get arbitrarily close to it, though.

Anyway, look here: http://en.wikipedia.org/wiki/Solar_sail

HallsofIvy
Homework Helper
"swans"? Is that a technical term.

"Light Sails" have long been a staple of science fiction. I don't know if NASA is doing any research but it would certainly been possible.

Why is the speed of light limited to 3*10^8 m/s? What is blocking it from going faster than that?

The speed of lights is an absolute maximum to speed, because it is the rate at which time flows. The digits it is expressed as are based on our units of measurement.

russ_watters
Mentor
The speed of light is not the rate at which time flows.

The speed of light is a dictated by the permeability and permittivity of space, as a consequence of Maxwell's equations.

prasannapakkiam
Well Watters has explained it clearly...

it is the rate at which time flows.

Not sure where I heard that...

Not sure where I heard that...

In a sense, c is the conversion rate between space and time. In many theoretical papers, "natural" units are adopted, where c = 1, \hbar = 1, etc. So that mass, length, time are all inter-convertible without changing the number.

The speed of light is a dictated by the permeability and permittivity of space, as a consequence of Maxwell's equations.

From the speed of light, did we derive the permeability and permittivity of space, or is it the other way?

Can anyone explain permeability and permittivity of space in simpler terms? I googled those keywords, but gave me pages with lots of greek symbols, which I used to understand in the past, for a short period in my life.

I would think that the definition of c, the speed of light, would rely somewhat on the Higgs field - that which endows mass upon particles. Massless particles travel at the speed of light, massive ones at less than c.

russ_watters
Mentor
From the speed of light, did we derive the permeability and permittivity of space, or is it the other way?
If I understand the history correctly, I think they were measured about the same time the first accurate measurements of the speed of light were being taken (by Michelson). So the speed of light was separately measured and calculated and found to be the same value.
Can anyone explain permeability and permittivity of space in simpler terms? I googled those keywords, but gave me pages with lots of greek symbols, which I used to understand in the past, for a short period in my life.
Basically, the resistance to passing magnetic and electric fields.

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